Most echo ultrasound imaging apparatus of the prior art generates images of structures within a body wherein the brightness of individual pixels corresponds to the magnitude of local discontinuities in the acoustic impedance at corresponding points in the body.
A new class of ultrasound imaging equipment, which produces images of structures within a body wherein the brightness of pixels corresponds to a local value of ultrasound attenuation at a corresponding point in the body have recently been described. U.S. Pat. No. 4,389,892 to Johathan Ophir and Nabil Maklad is incorporated herein, by reference, as background material. That patent describes apparatus which utilizes differential measurements of echo intensities along A-lines to estimate local tissue attenuation. A raster scan is used to generate images from such A-line measurements. U.S. Pat. No. 4,515,163 to Flax et al also describes a method for determining local tissue attenuation from the positions of zero crossings in A-line signals.
It is possible to directly determine the slope of the attenuation vs. frequency characteristic of a propagation medium from samples of a signal which has propagated through that medium. However, in the context of an ultrasound attenuation scanner, that method requires that a radio frequency ultrasound signal, which has propagated through the medium, be sampled at a rate which is at least twice as high as its highest frequency component in order to avoid aliasing error. This requires high speed sampling circuits which can significantly increase the cost and complexity of a scanner.